Method for forming a retrograde implant

ABSTRACT

A method of ion implantation is provided. The method comprising: providing a substrate; forming a masking image having a sidewall on the substrate; forming a blocking layer on the substrate and on the masking image; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer.

This application is a divisional of Ser. No. 10/083,062; filed on Feb.26, 2002 now U.S. Pat. No. 6,610,585.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor processing;more specifically, it relates to a method for forming a retrograde ionimplant.

BACKGROUND OF THE INVENTION

Modern semiconductor devices such as N channel field effect transistors(NFETs) and P-channel field effect transistors (PFETs) require carefultailoring of the dopant concentration profile in the channel region ofthe device in order to control voltage (V_(T)), off currents (I_(OFF))and short channel effects (SCE). For an NFET, the channel is formed bycontrol of the P-well dopant profile concentration. For a PFET, thechannel is formed by control of the N-well dopant profile concentration.Control of the respective N or P-well profile is accomplished byperforming at least one low-voltage and low-dose shallow ion implant andat least one high-voltage and high-dose ion retrograde implant, both ofthe same dopant type. A shallow implant is one in which the implantedspecies remain relatively close to the silicon surface. A retrogradeimplant is one in which the highest dopant concentration of theimplanted species occurs a distance below the silicon surface. Thechannel/well profile tailoring ion implant processes may be bestunderstood by reference to FIGS. 1A and 1B.

FIGS. 1A and 1B are partial cross-sectional views illustrating a relatedart method of forming a P-well or an N-well. In FIG. 1A, formed in asubstrate 100 is shallow trench isolation (STI) 105. Formed on a topsurface 110 of silicon substrate 100 is a thin oxide layer 115. Formedon a top surface 120 of STI 105 is a photoresist image 125. Alow-voltage and low-dose ion implantation of ion species “X,” where “X”represents boron for a P-well or phosphorus for an N-well, is performed.Ions 130A pass through thin oxide layer 115 and penetrate into substrate100 forming a shallow portion 135 of well 140. Ions 130B strikingphotoresist image 125 are absorbed by photoresist image 125. Ions 130C,striking near sidewall 145 of photoresist image 125 are deflected byatoms in the photoresist but image lack sufficient energy to passthrough the sidewall of the photoresist image.

In FIG. 1B, a high-voltage and high-dose ion implantation of ion species“X,” where “X” represents boron or for a P-well or phosphorus for anN-well, is performed. Ions 150A pass through thin oxide layer 115 andpenetrate into substrate 100 forming a deep portion 155 of well 135.Ions 150B striking photoresist image 125 are absorbed by the photoresistimage. Ions 150C, striking near sidewall 145 of photoresist image 125penetrate into the photoresist image, are deflected by atoms inphotoresist image 125, and have sufficient energy to escape throughsidewall 145, pass through thin oxide layer 115 and penetrate into anedge region 160 of well 140. Edge region 160 extends a distance “W” intowell 140 measured from resist sidewall 145. Edge region 160 extends adepth “D” measured from a top surface 165 of thin oxide layer 115.Obviously P-wells or N-wells away from photoresist image 125 are noteffected and do not have edge regions, “D” can range from about nearzero to 0.5 microns and “W” can range from about near zero to 1.2microns. The V_(T) of NFETs and PFETs devices fabricated in wellsadjacent to photoresist image 125 can differ from the V_(T) of NFETs andPFETs fabricated in wells away from (non-adjacent) by as much as about20 to 120 millivolts. The concentration of dopant in the shallow portion135 of well 140 in edge region 160 can be ten times the concentration ofdopant in the rest of shallow portion 135 of well 140.

Since devices fabricated away from edge region 160 or in wells away froma resist sidewall, which will not have an edge region, their V_(T) willnot be increased. Integrated circuits fabricated from a mix of edge andnon-edge NFETs and PFETs will have some slow devices and some fastdevices. Integrated circuits fabricated from a mix of edge and non-edgeNFETs and PFETs and will often exhibit asymmetric behavior.

Therefore, what is needed is a method of forming retrograde ion implantsthat dose not cause increased dopant concentrations in edge regions ofP-wells and N-wells.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method of ion implantationcomprising: providing a substrate; forming a masking image having asidewall on the substrate; forming a blocking layer on the substrate andon the masking image; and performing a retrograde ion implant throughthe blocking layer into the substrate, wherein the blocking layersubstantially blocks ions scattered at the sidewall of the maskinglayer.

A second aspect of the present invention is a method of ion implantationcomprising: providing a substrate; forming blocking layer on thesubstrate; forming a masking image having a sidewall on the blockinglayer; and performing a retrograde ion implant through the blockinglayer into the substrate, wherein the blocking layer substantiallyblocks ions scattered at the sidewall of the masking layer.

A third aspect of the present invention is a method of ion implantationcomprising: providing a substrate; forming a first blocking layer on thesubstrate and a second blocking layer on the first blocking layer;forming a masking image having a sidewall on the second blocking layer;and performing a retrograde ion implant through the first and secondblocking layer into the substrate, wherein the second or first andsecond blocking layers substantially blocks ions scattered at thesidewall of the masking layer.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIGS. 1A and 1B are partial cross-sectional views illustrating a relatedart method for forming a P-well or an N-well;

FIG. 2 is a flowchart of processing steps for forming a P-well or anN-well according to a first embodiment of the present invention;

FIGS. 3A and 3B are partial cross-sectional views illustrating the ionimplant steps of FIG. 2;

FIG. 4 is a flowchart of processing steps for forming a P-well or anN-well according to a second embodiment of the present invention;

FIGS. 5A and 5B are partial cross-sectional views illustrating the ionimplant steps of FIG. 4;

FIG. 6 is a flowchart of processing steps for forming a P-well or anN-well according to a third embodiment of the present invention;

FIGS. 7A and 7B are partial cross-sectional views illustrating the ionimplant steps of FIG. 6;

FIG. 8 is a flowchart of processing steps for forming a P-well or anN-well according to a fourth embodiment of the present invention; and

FIGS. 9A and 9B are partial cross-sectional views illustrating the ionimplant steps of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

A low-voltage ion implant generally results in a shallow ion implant.Shallow implants are often performed at low dose as well as low voltage.In a shallow ion implant, the implanted species remain relatively closeto the substrate surface and the highest dopant concentration of theimplanted species occurs at or very near the substrate surface. In thisdisclosure ion implants performed at a voltage of less than about 100Kev and at a dose of less than about 5E13 atoms/cm² are consideredshallow ion implants.

A high-voltage ion implant generally results in a retrograde ion implantprovided any blocking layer is sufficiently thin. Retrograde ionimplants are often performed at high-dose as well as high voltage. In aretrograde ion implant the highest dopant concentration of the implantedspecies occurs a distance below the substrate surface. In thisdisclosure ion implants performed at a voltage of equal to or greaterthan about 100 Kev and at a dose of about equal to or greater than 5E13atoms/cm². The present invention is also applicable to shallow ionimplants of low-energy and high-dose as well as to retrograde implantsof high-energy and low-dose though the dopant concentration of theshallow portion of a P-well or an N-well formed by a shallow high-doseion implant well would not be effected as much by scattering from aretrograde low-dose ion implant.

It has been determined that the amount of ion scattering of high-voltageand high-dose ion implants of boron and phosphorus is about the same forion incident angles in the range of about 0° to 10° and increasesignificantly above about 10° with boron scattering more thanphosphorus.

The ion implantation steps, both low-energy and low dose and high-energyand high-dose for all embodiments of the present invention, areperformed at an incident angle between about 0° to 10° with 7° beingmost commonly used, though the invention is applicable to any anglebetween 0° and 90°. The incident angle is measured from a line normal tothe surface being implanted.

While the present invention will be described in terms of a retrogradeboron or phosphorus implant to form either a P-well or an N-wellrespectively, the invention is equally applicable to a retrogradeimplant of other ion species containing atoms of arsenic, germanium orindium used alone or in combination with each other and in combinationwith boron and/or phosphorus. Also one skilled in the art would realizethat ion species containing boron or phosphorus could be implanted, forexample, BF₂ ⁺, and that the terms boron and phosphorus are intended toinclude all ion species containing boron or phosphorus.

The present invention is also applicable to other substrates such assapphire, ruby, SiGe and silicon-on-insulator (SOI).

FIRST EMBODIMENT

Referring to FIGS. 2, 3A and 3B, FIG. 2 is a flowchart of processingsteps for forming a P-well or an N-well according to a first embodimentof the present invention and FIGS. 3A and 3B are partial cross-sectionalviews illustrating the ion implant steps of FIG. 2. Referring to FIG,3A, in step 170 of FIG. 2, STI 105 is formed in substrate 100 and thinoxide layer 115 formed on top surface 110 of the silicon substrate.Depending upon the technology, thin oxide layer 115 may be explicitlyformed or may be formed as a result of the shallow trench isolation(STI) processes previously performed. In one example, thin oxide layer115 is about 40 to 60 Å thick. Both STI 105 and thin oxide layer 115 areoptional.

Referring to FIG. 3A, in step 175 of FIG. 2, photoresist image 125 isformed on top surface 120 of STI 105 by any one of a number ofphotolithographic methods known to one skilled in the art. While theexample of a photoresist image is used, other masking images formed frommasking layers comprised of materials other than photoresist may beemployed in this and subsequent embodiments of the present invention. Inone example, photoresist image 125 is either positive or negativephotoresist and is about 0.8 to 2.2 microns thick.

Referring to FIG. 3A, in step 180 of FIG. 2, a low-voltage and low-doseion implantation of ion species “X,” where “X” represents boron or for aP-well or phosphorus for an N-well, is performed. Ions 130A, strikingthin oxide layer 115 pass through the thin oxide layer and penetrateinto substrate 100 forming shallow portion 135 of well 140. Ions 130Bstriking photoresist image 125 are absorbed by the photoresist image.Ions 130C, striking photoresist image 125 near sidewall 145 of thephotoresist image pass into the photoresist image and are deflected byatoms in the photoresist image. Ions 130C lack sufficient energy toescape through sidewall 145 of photoresist image 125 or if they doescape, to pass through thin oxide layer 115.

Referring to FIG. 3B, in step 185 of FIG. 2, a blocking layer 190 isformed over thin oxide layer 115 and photoresist image 125. It is notnecessary that blocking layer cover sidewall 145 of photoresist image125. Of course, when blocking layer 190 covers sidewall 145, thepossibility exists for scattering of ions off the blocking layer itself,so the thickness of the blocking layer needs to take this into accountas well.

Referring to FIG. 3B, in step 195 of FIG. 2, a high-voltage andhigh-dose ion implantation of ion species “X,” where “X” representsboron or for a P-well or phosphorus for an N-well, is performed. Ions150A striking blocking layer 190 pass through the blocking layer andthrough thin oxide layer 115 and penetrate into substrate 100 formingdeep portion 155 of well 140. Ions 150B striking blocking layer 190,pass through the blocking layer, penetrate into photoresist image 125and are absorbed by the photoresist image. Ions 150C striking blockinglayer 190 near sidewall 145 of photoresist image 125 pass through theblocking layer, penetrate into the photoresist image and are deflectedby atoms in the photoresist image. Ions 150C have sufficient energy topass through sidewall 145 of photoresist image 125 but not throughblocking layer 190 and are absorbed by the blocking layer.

A blocking layer substantially blocks ions scattered at the sidewall ofa masking image from penetrating into the substrate by absorbing asignificant portion of the scattered ions alone or in combination withoverlaying or underlaying layers. Substantial blocking may be determinedto have occurred when little or no difference in the V_(T) of edgedevices and the V_(T) of non-edge devices can be measured or when thedifference in edge device V_(T) and non-edge device V_(T) is within apreset limit. Alternatively, substantial blocking may be determined tohave occurred when under similar processing conditions except for thepresence or absence of a blocking layer, the V_(T) of edge devicesfabricated without the use of a blocking layer is measurably different(or different within a preset limit) from the V_(T) of edge devicesfabricated with the use of a blocking layer. Secondary ion massspectroscopy (SIMS) analysis may also be used by comparing structuresimplanted away from resist edges with structures implanted near or nextto resist edges.

That a given layer will exhibit substantial blocking can also bepredicted by combining a theoretical determination of the amount ofenergy remaining to deflected ions with data from range tables orcalculations using range equations of the material and thickness of theblocking layer such that a predetermine percentage of the total numberof deflected ions do not penetrate into the substrate.

Blocking layer 190 must be thin enough to allow ions 150A to passthrough but thick enough to block ions 150C from passing through, ions150C having lost energy by collisions with atoms within photoresistimage 125. In one example, blocking layer 190 is formed from any one ofseveral organic anti-reflective coating (ARC) materials or otherconformal materials well known in the art and is about 900 to 3600 Åthick.

Referring to FIG. 3B, in step 200 of FIG, 2, resist image 125 andblocking layer 190 are removed.

SECOND EMBODIMENT

Referring to FIGS. 4, 5A and 5B, FIG. 4 is a flowchart of processingsteps for forming a P-well or an N-well according to a second embodimentof the present invention and FIGS. 5A and 5B are partial cross-sectionalviews illustrating the ion implant steps of FIG. 4. Referring to FIG,5A, in step 205 of FIG. 4, STI 105 is formed in substrate 100 and thinoxide layer 115 formed on top surface 110 of the silicon substrate. Inone example, thin oxide layer 115 is about 40 to 60 Å thick. Both STI105 and thin oxide layer 115 are optional.

Referring to FIG. 5A, in step 210 of FIG. 4, a blocking layer 215 isformed over thin oxide layer 115 and STI 105. In one example, blockinglayer 215 is an organic material such as polyimide or photoresist and isabout 1000 to 3000 Å thick.

Referring to FIG. 5A, in step 220 of FIG. 4, photoresist image 125 isformed on a top surface 225 of blocking layer 215. Photoresist image 125is aligned over STI 105. Photoresist image 125 may be formed by any oneof a number of photolithographic methods known to one skilled in theart. In one example, photoresist image 125 is either positive ornegative photoresist and is about 0.8 to 2.0 microns thick.

If blocking layer 215 is formed from a photoresist material thenphotoresist image 125 is formed from a photoresist of opposite polarityfrom that of the blocking layer. For example, if blocking layer 215 isformed from positive resist, then photoresist image 125 is formed fromnegative resist. If blocking layer 215 is formed from negative resist,then photoresist image 125 is formed from positive resist.

Referring to FIG. 5A, in step 230 of FIG. 4, a high-voltage andhigh-dose ion implantation of ion species “X,” where “X” representsboron or for a P-well or phosphorus for an N-well, is performed. Ions150A striking blocking layer 215 pass through the blocking layer,through thin oxide layer 115 and penetrate into substrate 100 formingdeep portion 155 of well 135. Ions 150B striking blocking layer 215,pass through the blocking layer, penetrate into photoresist image 125and are absorbed by the photoresist image. Ions 150C, striking blockinglayer 215 near sidewall 145 of photoresist image 125 pass throughblocking the layer, are deflected by atoms in the photoresist image andhave sufficient energy to pass through sidewall 145 of the photoresistimage but not through the blocking layer and are absorbed by theblocking layer.

Blocking layer 215 must be thin enough to allow ions 150A to passthrough but thick enough to block ions 150C from passing through, ions150C having lost energy by collisions with atoms within photoresistimage 125.

Referring to FIG. 5B, in step 235 of FIG. 4, blocking layer 215 (seeFIG. 5A) is thinned to form a thinned portion 215A of blocking layer 215where the blocking layer is not protected by photoresist image 125. Inone example, thinned portion 215A of blocking layer 215 is about 0 to1000 Å thick and the thinning was accomplished by any one of well knownreactive ion etch (RIE) processes. Photoresist image 125 (see FIG. 5A)is also thinned by the RIE process to form thinned photoresist image125A, so it is the combination of the thickness of thinned portion 215Aof blocking layer 215 and the thickness of thinned photoresist image215A that must be sufficient to block low voltage ion 130A.

Referring to FIG. 5B, in step 240 of FIG. 4, a low-voltage and low-doseion implantation of ion species “X,” where “X” represents boron or for aP-well or phosphorus for an N-well, is performed. Ions 130A, strikingthinned blocking layer 215A pass through the thinned blocking layer,pass through thin oxide layer 115 and penetrate into substrate 100forming shallow portion 135 of well 140. Ions 130B striking photoresistimage 125 are absorbed by the photoresist image. Ions 130C, strikingphotoresist image 125 near sidewall 145 of the photoresist image aredeflected by atoms in the photoresist image but lack sufficient energyto escape the photoresist image or if they do escape, to penetratethinned portion 215A of blocking layer 215.

Referring to FIG. 5B, in step 245 of FIG. 4, resist image 125 thinnedportion 215A and blocking layer 215 are removed.

THIRD EMBODIMENT

Referring to FIGS. 6, 7A and 7B, FIG. 6 is a flowchart of processingsteps for forming a P-well or an N-well according to a third embodimentof the present invention and FIGS. 7A and 7B are partial cross-sectionalviews illustrating the ion implant steps of FIG. 6. Referring to FIG,7A, in step 250 of FIG. 6, STI 105 is formed in substrate 100 and thinoxide layer 115 formed on top surface 110 of the silicon substrate. Inone example, thin oxide layer 115 is about 40 to 60 Å thick. Both STI105 and thin oxide layer 115 are optional.

Referring to FIG. 7A, in step 255 of FIG. 6, a blocking layer 260 isformed over thin oxide layer 115 and STI 105. In one example, blockinglayer 260 is formed from silicon oxide, silicon nitride, polysilicon,borosilicate glass (BSG), boro-phosphorus-silicate glass (BPSG), quartz,tetraethoxysilane (TEOS) oxide or high density plasma (HDP) oxide and isabout 200 to 3600 Å thick.

Referring to FIG. 7A, in step 265 of FIG. 6, photoresist image 125 isformed on a top surface 270 of blocking layer 260. Photoresist image is125 is aligned over STI 105. Photoresist image 125 may be formed by anyone of a number of photolithographic methods known to one skilled in theart. In one example, photoresist image 125 is either positive ornegative photoresist and is about 1.2 to 2.2 microns thick.

Referring to FIG. 7A, in step 275 of FIG. 6, a high-voltage andhigh-dose ion implantation of ion species “X,” where “X” representsboron or for a P-well or phosphorus for an N-well, is performed. Ions150A striking blocking layer 260 pass through the blocking layer andthrough thin oxide layer 115 and penetrate into substrate 100 formingdeep portion 155 of well 140. Ions 150B striking resist image 125,penetrate into the photoresist image and are absorbed by the photoresistimage. Ions 150C, striking photoresist image 125 near sidewall 145 ofthe photoresist image penetrate into the photoresist image, aredeflected by atoms in the photoresist image and have sufficient energyto pass through sidewall 145 of the photoresist image. Ions 150C do nothave sufficient energy to pass through blocking layer 260 and areabsorbed by the blocking layer.

Blocking layer 260 must be thin enough to allow ions 150A to passthrough but thick enough to block ions 150C from passing through, ions150C having lost energy by collisions with atoms within photoresistimage 125.

Referring to FIG. 7B, in step 280 of FIG. 6, portions of blocking layer260 not protected by resist image 125 are removed.

Referring to FIG. 7B, in step 2985 of FIG. 6, a low-voltage and low-doseion implantation of ion species “X,” where “X” represents boron or for aP-well or phosphorus for an N-well, is performed. Ions 130A, strikingthin oxide layer 115 pass through the thin oxide layer and penetrateinto substrate 100 forming shallow portion 135 of well 140. Ions 130Bstriking photoresist image 125 are absorbed by the photoresist image.Ions 130C, striking photoresist image 125 near sidewall 145 of thephotoresist image are deflected by atoms in the photoresist image butlack sufficient energy to escape the photoresist image or if they doescape, to penetrate thin oxide layer 115.

Referring to FIG. 7B, in step 290 of FIG, 6, resist image 125 andblocking layer 260 are removed.

FOURTH EMBODIMENT

Referring to FIGS. 8, 9A and 9B, FIG. 8 is a flowchart of processingsteps for forming a P-well or an N-well according to a fourth embodimentof the present invention and FIGS. 9A and 9B are partial cross-sectionalviews illustrating the ion implant steps of FIG. 7. Referring to FIG.9A, in step 295 of FIG. 8, STI 105 is formed in substrate 100 and thinoxide layer 115 formed on top surface 110 of the silicon substrate. Inone example, thin oxide layer 115 is about 40 to 60 Å thick. Both STI105 and thin oxide layer 115 are optional.

Referring to FIG. 9A, in step 300 of FIG. 8, a first blocking layer 305is formed over thin oxide layer 115 and STI 105 and a second blockinglayer 310 is formed on top surface 315 of first blocking layer 305. Inone example, first blocking layer 305 is formed from silicon nitride orpolysilicon and is 100 to 500 Å thick and second blocking layer 310 isformed from borosilicate glass (BSG), boro-phosphorus-silicate glass(BPSG), quartz, tetraethoxysilane (TEOS) oxide, high density plasma(HDP) oxide or polysilicon and is about 500 to 2500 Å thick.

Referring to FIG. 9A, in step 320 of FIG. 8, photoresist image 125 isformed on a top surface 325 of second blocking layer 310. Photoresistimage is 125 is aligned over STI 105. Photoresist image 125 may beformed by any one of a number of photolithographic methods known to oneskilled in the art. In one example, photoresist image 125 is eitherpositive or negative photoresist and is about 1.2 to 2.2 microns thick.

Referring to FIG. 9A, in step 330 of FIG. 8, a high-voltage andhigh-dose ion implantation of ion species “X,” where “X” representsboron or for a P-well or phosphorus for an N-well, is performed. Ions150A striking second blocking layer 310 pass through second blockinglayer, pass through first blocking layer 305, pass through thin oxidelayer 115 and penetrate into substrate 100 forming deep portion 155 ofwell 140. Ions 150B striking resist image 125, penetrate into thephotoresist image and are absorbed by the photoresist image. Ions 150C,striking photoresist image 125 near sidewall 145 of the photoresistimage penetrate into the photoresist image, are deflected by atoms inthe photoresist image, have sufficient energy to pass through sidewall145 of the photoresist image but not through second blocking layer 310or first and second blocking layer 305 and 310 and are absorbed by theblocking layer(s).

First and second blocking layers 305 and 310 must be thin enough toallow ions 150A to pass through but thick enough to block ions 150C frompassing through, ions 150C having lost energy by collisions with atomswithin photoresist image 125.

Referring to FIG. 9B, in step 335 of FIG. 8, portions of second blockinglayer 340 not protected by resist image 125 are removed. First blockinglayer 305 acts as an etch stop during the etching of second blockinglayer 310.

Referring to FIG. 9B, in step 340 of FIG. 8, a low-voltage and low-doseion implantation of ion species “X,” where “X” represents boron or for aP-well or phosphorus for an N-well, is performed. Ions 130A, strikingfirst blocking layer 305, pass through first blocking layer 305, passthrough thin oxide layer 115 and penetrate into substrate 100 formingshallow portion 135 of well 140. Ions 130B striking photoresist image125 are absorbed by the photoresist image. Ions 130C, strikingphotoresist image 125 near sidewall 145 of the photoresist image aredeflected by atoms in the photoresist image but lack sufficient energyto escape the photoresist image or if they do escape, to penetrate firstblocking layer 305.

Referring to FIG. 9B, in step 345 of FIG, 8, resist image 125, secondblocking layer 310 and first blocking layer 305 are removed.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method of ion implantation comprising:providing a substrate having a shallow trench isolation; forming a firstblocking layer on said substrate having a shallow trench isolation and asecond blocking layer on said first blocking layer; forming a maskingimage having a sidewall on said second blocking layer; and performing aretrograde ion implant into said masking image and through said firstand second blocking layers into said substrate, said first and secondblocking layers having a sufficient combined thickness to substantiallyblock from entering said substrate ions scattered in said masking imagefrom said retrograde ion implant and exiting said masking image throughthe sidewall of said masking image.
 2. The method of claim 1, furthercomprising: after said performing the retrograde ion implant, thinningor removing said second blocking layer where said second blocking layeris not covered by said masking image; and performing a shallow ionimplant.
 3. The method of claim 1, wherein said first and secondblocking layers block a sufficient quantity of said ions of saidretrograde ion implant so that a threshold voltage of a field effecttransistor formed in said substrate immediately adjacent to saidsidewall of said masking image about equal to a threshold voltage of afield effect transistor formed in said substrate away from said sidewallof said masking image.
 4. The method of claim 1, wherein said performingthe retrograde implant is performed using ion species containing atomsselected from the group consisting of boron, phosphorous, arsenic,germanium and indium.
 5. The method of claim 1, wherein said performingthe retrograde ion implant is performed at an energy of equal to orgreater than 100 Kev.
 6. The method of claim 1, further comprising:performing a shallow ion implant after removing only said secondblocking layer.
 7. The method of claim 2, wherein said performing theshallow ion implant is performed using ion species containing atomsselected from the group consisting of boron, phosphorous, arsenic,germanium and indium.
 8. The method of claim 2, wherein said performingthe shallow ion implant is performed at an energy of less than 100 Kevand a dose of less than 5E13 atm/cm².
 9. The method of claim 1, whereinsaid first blocking layer includes a first material selected from thegroup consisting of silicon nitride and polysilicon and, wherein saidsecond blocking layer includes a second material selected from the groupconsisting of borosilicate glass, boro-phosphorus-silicate glass,quartz, tetraethoxysilane oxide, high density plasma oxide polysiliconand silicon nitride.